The present invention relates to electric storage batteries and, more particularly, this invention relates to an edge seal for a battery case containing swellable separators.
Proper operation of a battery requires electrical insulation between the positive and negative electrodes of a battery. If there is a slightest breach, the battery will discharge or will slowly short out with resultant premature loss of capacity. Separators are utilized to separate the electrodes. A battery assembly containing the edges of the separator must mate properly with case components in order to form a seal that insulates mass flow between different electrode compartments. Typically the case components tend to be either stainless steel cans, though increasingly these components can be made of nonmetal materials such as durable plastics. Separator materials tend to be regenerated cellulose or semi-porous polyolefin films modified to be ionically conducting. Thus the need arises of sealing polymer separator to either a metal housing or a plastic housing.
U.S. Pat. No. 5,057,385, Hope et al.
U.S. Pat. No. 4,209,574, Ruetschi
U.S. Pat. No. 4,172,183, Ruetschi
U.S. Pat. No. 4,121,021, Ogawa et al.
U.S. Pat. No. 3,884,723, Wuttke.
U.S. Pat. No. 3,708,343, Walsh.
U.S. Pat. No. 3,489,616, Fangradt et al.
U.S. Pat. Nos. 5,272,020 uses a hot melt thermoplastic sealant at the bottom edge zone of the separator system which impregnates the separator system at the edges zone. U.S. Pat. No. 5,462,819 discloses a hot melt material which is metered into the cell so that the hot melted material flows under the bottom edge of the separator, which is then pushed down and seated in the hot melt material as it cools. The end result is a barrier which forms at the bottom of the cell and at both sides of the separator. U.S. Pat. No. 6,099,987 communicates a cup seal made of micro-porous or non-porous membrane, attached to the bottom with a small amount of hot sealant, with the aim of minimizing the amount of hot-sealant used in the seal.
In U.S. Pat. No. 4,209,574 and 4,172,183 Ruetschi designs a metal case that pushes against separators which are then pushed into a sealing ring made of nylon, neoprene or teflon. Ogawa et al. in U.S. Pat. No. 4,121,021 demonstrate a metal case pushing on an annular gasket, which in turn compresses the separators. Walsh in U.S. Pat. No. 3,708,343 has the top of the battery compressing on a grommet, and in this state the battery is sealed with epoxy. Gordy in U.S. Pat. No. 5,198,314 uses polyphenelyne sulfide as seal material.
As mentioned in U.S. Pat. No. 6,099,987 hot-melt sealants diminish the capacity of the battery and lower the volumetric density. In our invention we avoid the use of hot-melt sealants altogether by relying on a mechanism where the battery seals itself. In our invention we also do not rely on applying direct compressional force on the separators to effect a seal. Rather we rely on the swelling properties of cellulose in the presence of an alkali hydroxide to provide a tight seal between the two battery compartments.
Cellulose in the form of cellophane has been a widely used separator for a variety of alkaline batteries. The reasons include its low electrical impedance as well as excellent ion transport in alkaline environments. Cellulose-alkali hydroxide interactions in aqueous solutions are known to be strong and asymmetrical. Swelling of cellulose material in the presence of alkali hydroxides occurs on a time scale of seconds to minutes. The swelling power depends on the alkali hydroxide concentration, with a pronounced change in the X-ray lattice dimensions above a certain lye concentration. It also depends on the cellulose starting material. For cellophane, a 1 mil thick film swells to at least 2 times its original thickness when exposed to an aqueous potassium hydroxide solution (50% by weight) for more than a few seconds.
A battery seal is disclosed whereby a cellulose-based separator or separators, placed between undulating matching edges of a mating case halves suitably made of thermoplastic resin and occupying, in dry form, at least 80% of available edge space, swells by at least 20% from its dry thickness in the presence of an alkali-hydroxide of specific gravity 1.25 to 1.45 and thus generates a tight seal insulating the electrode compartments of the battery.
The edge sealing system according to the invention utilizes the profile of the edges of 2 mating casing members to grip and seal the swellable separators. One edge is formed to contain at least 2 convex undulations. The edge of the opposed casing members contains the same number of mating concave grooves. Spacing means are present to space the convex undulations from the grooves an amount at least equal to the thickness of the edges of a stack of swellable separators in the dry state. Typically, the dry thickness of the separators occupy from at least 80% but no more than 98%, preferably 90-95% of the space between the undulations and the grooves. When the separators absorb electrolyte they expand and are locked between the mating surfaces. The undulations are preferably rounded to prevent cutting or tearing the separators.
The outer edge of the space must be sealed to prevent leakage of electrolyte from the battery case. A preferred method of sealing the edge is to form a rim on the outer edge of one half of the case which projects across the outer opening of the space that grips the edges of the separators. The opposed case member has a groove that receives the rim. The length of the groove is less than that of the rim, a predetermined distance forming a limit or stop that defines the thickness of the separator gripping space. A recess can be formed at the junction of the rim and groove to receive a sealant or potting material such as an epoxy. The electrolyte is injected into the separators after the recess is sealed.
The expanded separators are reliably gripped by the undersized channel. The rounded undulations firmly grip the edges of the separators without ripping, puncturing or tearing the separators. The rim and grooves seal the channel and simultaneously space the edges a fixed distance apart. The casing is readily manufactured by molding of resin materials at moderate cost.
These and many other features and attendant advantages of the invention will become apparent as the invention becomes better understood by reference to the following detailed description when considered in conjunction with the accompanying drawings.